Many detection systems for determining the presence or absence of a particular target analyte in a sample are known. Examples of detection systems for detecting analytes include immunoassays, such as an enzyme linked immunosorbent assays (ELISAs), which are used in numerous diagnostic, research and screening applications. Generally, these detection systems detect the target analyte when it binds to a specific binding agent or probe resulting in a measurable signal.
When using known detection systems, such as immunoassays, the ability to detect a target analyte is often limited by the low concentration of the target analyte in the sample, non-specific binding of the target analyte and background interference generated by other molecules or substances in the sample. The ability to detect a target analyte in a sample taken from biological materials is often limited by most, if not all, of these factors.
Target analytes present in a sample are difficult to detect because the number of potential analytes capable of generating a signal is often limited. A common solution to this problem is to amplify the target analyte using polymerase chain reaction (PCR). However, PCR is only available for nucleic acid analytes and the method takes at least an hour to produce enough of the target analyte to generate a detectable signal in most systems.
Additionally, detection of short nucleic acids, such as micro-RNA molecules is difficult to achieve using PCR. Micro-RNA molecules are shorter (˜22 nucleotides) than the combined size of regular PCR primers (˜40 nucleotides) and therefore cannot be detected using a standard PCR reaction. A number of strategies exist to circumvent this problem, such as ligating DNA molecules and conducting PCR amplification on the enlarged fragment or hybridizing the target to radiolabeled probes to detect them without PCR amplification. However, these alternatives are time consuming, labor intensive, require specialized reagents or equipment, and/or have low sensitivity.
The presence of other molecules in the sample can also interfere with the detection of the target analyte by producing background noise. For example, in systems where the detection probe is bound to a surface, a common source of signal interference is the non-specific interaction between molecules in the sample and the surface surrounding the probe.
Purification of a sample is often performed to remove the undesired interfering molecules from the sample in order to better detect the target analyte. However, purification is time consuming, often results in a reduction in the amount of the target analyte in the sample and can alter the concentration of the target analyte in the sample by diluting or concentrating the sample.
Recently, methods and detection systems to selectively detect the presence of a target analyte have been developed, such as those disclosed in U.S. Pub. No. 2009/0011946. In another method, micromechanical devices are used as sensors for detecting physical or chemical changes caused by chemical interactions between natural bio-polymers, which are non-identical binding partners, where one binding partner or probe molecule is placed on a cantilever for possible reaction with a sample analyte molecule. U.S. Pat. No. 6,436,647, “Method for Detecting Chemical Interactions Between Naturally Occurring Biological Analyte Molecules that are Non-Identical Binding Partners.” According to this method, a chemical analyte is detected by generating a physical or chemical change, whether through affinity binding, hydrogen bonding, electrostatic attractions, hydrophobic effects, dipole interactions or a heat reaction. The physical or chemical change induces stress on the cantilever which causes the cantilever to move or deflect, which is measured using methods commonly used for detecting cantilever deflection. However, this method requires a relatively high target concentration in the sample since cantilever deflection requires binding of a large number of target molecules. Additionally, the method is prone to non-specific interactions that produce background noise. Since signal generation occurs upon target binding to probes located on a surface, nonspecific binding to the surface can generate signal and background noise.
Accordingly, there is a need for a detection unit and systems of such units as well as methods capable of detecting very low concentrations of target analytes while reducing non-specific binding in the sample.